1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to an optical efficiency enhancing film and an LCD using the same.
2. Discussion of the Related Art
Today, as our society rapidly advances to information-oriented society, a flat panel display having advantages of slimness, lightweight, and low power consumption is required.
The LCD is one of the flat panel displays having advantages of a superior visibility, a low power consumption and a low heat radiation in comparison with the cathode ray tube (CRT) having the same screen size. Because of the above advantages, the LCD is widely applied to hand-held terminals, computer monitors and televisions along with the plasma display panel (PDP) and the field emission display (FED), and is expected to be a next generation display.
The LCD includes two substrates each having an electrode for generating an electric field and facing each other, and a liquid crystal layer interposed therebetween. Then, a predetermined voltage is supplied to the electric field electrodes to generate electric field between the electric field electrodes so that liquid crystal molecules may be controlled by the generated electric field. As a result, optical transmission rate of the liquid crystal material can be adjusted to thereby display images.
In general, the liquid crystal molecules are anisotropically aligned. The anisotropy of a liquid crystal cell or a film consisting of the liquid crystal molecules is affected by distribution degree of the liquid crystal molecules and distribution degree tilt angles of the liquid crystal molecules with respect to a substrate. Such properties of the liquid crystal are a main factor to change polarization of light depending on a viewing angle of the liquid crystal cell or the film.
FIG. 1 illustrates a general LCD according to the related art.
In FIG. 1, an LCD includes a lower substrate 120, an upper substrate 130, a liquid crystal layer 140, a first polarization plate 129, and a second polarization plate 139. The lower substrate 120 is provided with thin film transistors (TFTs) (not shown) arranged in matrix configuration and pixel regions defined by gate lines and data lines (not shown). The upper substrate 130 corresponds to the lower substrate 120 and is provided with a color filter layer (not shown) consisting of a plurality of color filter patterns to represent colors. The liquid crystal layer 140 is made by filling liquid crystal between the upper substrate 130 and lower substrate 120. The first polarization plate 129 is attached to a lower surface of the lower substrate 120, and passes and linearly polarizes natural light. The second polarization plate 139 is attached to an upper surface of the upper substrate 130 and has an optical transmission axis perpendicular to that of the first polarization plate 129.
The LCD further includes a backlight unit 110 for supplying the liquid crystal panel with the light generated from a light source 111. At this time, the backlight unit 110 is provided on an outer side of the lower substrate 120.
For example, although not illustrated in FIG. 1, the lower substrate of the twisted nematic (TN) mode LCD (the lower substrate 120) includes a transparent substrate upon which gate lines and data lines (not shown) are formed to cross over each other, a gate electrode extending from the gate line, a gate insulating layer formed on an entire surface including the gate electrode, a semiconductor layer formed on the gate insulating layer, a thin film transistor formed on the semiconductor layer and having source/drain electrodes, and a passivation layer formed on the transparent substrate upon which the thin film transistor is formed, and a pixel electrode connected to the drain electrode through a contact hole formed on the passivation layer.
The upper substrate 130 includes a transparent substrate, a black matrix (BM) formed on the transparent substrate to prevent light from being transmitted through the regions other than the pixel electrodes, red, green and blue color filter patterns formed on the BM to represent colors, and a common electrode on the color filter patterns.
The first and second polarization plates 129 and 139 are formed on outer surfaces of the lower and upper substrate 120 and 130 respectively such that the optical transmission axes thereof may be perpendicular to each other. Natural light polarized to single polarized light is transmitted through the polarization plates but other polarized light is absorbed or scattered.
Generally, light is an electromagnetic wave and the oscillation direction is perpendicular to its traveling direction. Polarized light is a light whose oscillation direction is polarized, that is, a light oscillating strongly in a specific direction among the direction perpendicular to its traveling direction.
Accordingly, the light emitted from the backlight unit 110 provided on the lower portion of the liquid crystal panel oscillates in all the directions perpendicular to its traveling direction with the same probability. The first and second polarization plates 129 and 139 pass only the light that oscillates in the direction identical to the polarization axis thereof, but absorb or scatter, using a proper medium, the light that oscillates in the direction different from the polarization axis thereof, so that only the light that oscillates in a specific direction is transmitted through the polarization plates 129 and 139.
To make the polarization axes over and below the liquid crystal layer 140 be perpendicular to each other, the polarization plates 129 and 139 are attached to the outer surfaces of the lower and upper substrates 120 and 130 respectively. Then, the strength of the transmitted light is adjusted according to the rotation degree of polarization axis of the liquid crystal layer 140 such that grays between black and white colors may be represented.
However, in the LCD of related art, the light transmitted through and polarized by the first polarization plate 129 generates non-polarized light in passing the inside of the liquid crystal panel. For example, the light is scattered by the step portion formed on the lower substrate 120 and the color filter layer 133 formed on the upper substrate 130. Thus, some of the light polarized by the first polarization plate 129 is changed into the non-polarized light due to such scatterings.
Accordingly, the optical transmission rate of the LCD of the related art is dramatically lowered due to the non-polarized light so that the brightness is lowered.